Superabsorbers are water-insoluble, cross-linked polymers which, with swelling and the formation of hydrogels, are able to absorb, and retain at a certain pressure, large quantities of aqueous liquids and body fluids such as, for example, urine or blood. As a result of these characteristic properties these polymers are incorporated in sanitary articles such as, for example, babies' nappies, incontinence products or sanitary towels, by way of a principal application.
The superabsorbers which are currently available commercially are substantially cross-linked polyacrylic acids or cross-linked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralised with caustic soda or caustic potash solution.
The increasing tendency towards increasingly small, thin sanitary articles such as babies' nappies, incontinence products and sanitary towels arises from aesthetic and environmental considerations. In order to guarantee that the total retention capacity of the sanitary articles remains unchanged, this requirement can be met only by reducing the proportion of bulky fluff. As a result, the superabsorber is required to serve further functions in terms of transportation and distribution of liquid, which can be summarized as permeability properties.
In the case of superabsorber materials permeability is understood to mean the ability, in the swollen state, to transport liquids which have been added and to distribute them three-dimensionally. This process takes place in the swollen superabsorber gel by way of capillary transportation through interstices between the gel particles. Transportation of liquid through swollen superabsorber particles themselves is subject to the laws of diffusion and is a very slow process which plays no part in the distribution of liquid in the situation in which the sanitary article is used. In the case of superabsorber materials which owing to inadequate gel stability cannot bring about capillary transportation, a separation of the particles from one another was ensured by embedding these materials in a fiber matrix, avoiding the phenomenon of gel blocking. In new-generation nappy constructions the absorber layer contains only little fiber material, or none at all, to support the transportation of liquid. The superabsorbers used here must accordingly be sufficiently stable in the swollen state for the swollen gel still to have an adequate quantity of capillary spaces through which liquid can be transported.
In order to obtain superabsorber materials having high gel strength, the degree of cross-linking of the polymer can, on the one hand, be increased, which necessarily results in a reduction in swelling ability and retention capacity. An optimized combination of different cross-linking agents and comonomers, such as is described in Patent Specification DE 196 46 484, was admittedly able to improve the permeability properties, but not to a level which, for example, enables a layer which optionally comprises only superabsorbers to be built into in a nappy construction.
Methods of post-cross-linking the surface of the polymer particles can furthermore be used. In so-called post-cross-linking the carboxyl groups of the polymer molecules on the surface of the superabsorber particles are reacted with different post-cross-linking agents which are capable of reacting with at least two of the carboxyl groups close to the surface. As well as bringing about increased gel strength, the ability to absorb liquid at pressure is in particular greatly improved because the known phenomenon of gel blocking, in which polymer particles which have begun to stick together, thus preventing further liquid absorption is suppressed.
The surface treatment of liquid-absorbent resins is already known. Ionic complexing of the carboxyl groups close to the surface with polyvalent metal cations is proposed in U.S. Pat. No. 4,043,952 in order to improve dispersibility. The treatment takes place with salts of polyvalent metals which are dispersed in organic, optionally water-containing, solvents (alcohols and other organic solvents).
DE-A-40 20 780 describes a post-treatment of superabsorber polymers with reactive, surface-cross-linking compounds (alkylene carbonates) in order to increase the liquid absorption capability at pressure. DE-A-35 03 458 describes a surface post-cross-linking of superabsorbent polymers with polyfunctional cross-linking agents such as polyvalent metal compounds in the presence of inert, inorganic powder such as SiO2 in order to improve the absorption properties and create a non-adherent gel of the polymer particles.
According to the teaching of EP-A-0 574 260 superabsorbent polymers are obtained which have a low residual monomer content which does not change crucially even when surface cross-linking takes place, if certain conditions are observed in the polymerisation and the post-cross-linking is carried out with conventional polyfunctional cross-linking agents such as polyols, alkylene carbonates, polyvalent metal salts under conventional conditions. The post-cross-linked polymers show good absorption without the application of pressure.
According to EP-A-0 889 063 superabsorbent polymers which are preferably already surface-cross-linked can be equipped to resist radical degradation by body fluids, in particular L-ascorbic acid by post-treatment with a titanium or zirconium compound and a compound which chelates these metal compounds.
EP 0 233 067 describes water-absorbent resins which are cross-linked on the surface, which are obtained by the reaction of a superabsorbent polymer powder having from 1 to 40 wt. % of an aluminium compound, in relation to the polymer powder. A mixture of water and diols is used as the treatment solution, which should render the use of low alcohols as solvents superfluous. 100 parts by weight of cross-linking agent solution are preferably applied to from 100 to 300 parts by weight of absorber. The diols (for example polyethylene glycol 400 and 2000, 1,3-butanediol or 1,5-pentanediol) which are added to the reaction medium, water, also serve, inter alia, to prevent agglomeration of the superabsorber when treated with the large quantities of aqueous treatment solution which are used here. The solvent is removed in subsequent drying at 100° C. The properties of the polymers thus treated are inadequate, with no improvement in the absorbency at pressure being achieved. A treatment with large quantities of treatment solution is furthermore not economically practicable in modem continuous processes.
WO 96/05234 describes a process for the treatment of superabsorbent polymers, according to which the surface of the absorber particles, which contain at least 10 wt. % water, was equipped with a cross-linked layer obtained by a reaction of a reactive hydrophilic polymer or a reactive organometallic compound with an at least difunctional cross-linking agent at temperatures below 100° C. Metal salts are not listed. The metal compounds utilised must be capable of reacting with the functional groups of the cross-linking agent. Organometallic compounds which should be present in a ratio by weight of from 0.1 to 30 in relation to the cross-linking compound are therefore recommended as the metal compounds. The polymers obtained should have a balanced ratio of absorption, gel strength and permeability, with the measured values which are indicated being obtained under less critical conditions. Thus, for example, the absorption and the permeability are determined with no pressure load. A disadvantage of this known process is the use of solvents and toxicologically risky cross-linking reagents such as, for example, the polyimines, alkoxylated silane and titanium compounds and epoxides, which are cited as preferred.
According to the teaching of WO 95/22356 and WO 97/12575 an improvement in the permeability properties and liquid transportation properties is obtained by a corresponding treatment of commercially obtainable superabsorber polymers with amino polymers in organic solvents. The serious disadvantage of the process described here, apart from the use of toxicologically risky polyamines and polyimines, lies in the use of large quantities of organic solvents which are necessary for the treatment of the polymers. The issue of safety and the cost associated with this preclude large-scale industrial production. Apart from the toxicological risk posed by these treatment agents, consideration should furthermore be given to their additional tendency to decompose at the high post-cross-linking temperatures, which is manifested, inter alia, in a yellow coloration of the absorber particles.
In order to prepare water-absorbent polymers having better abrasion resistance, Japanese published patent application JP-A-09124879 teaches surface post-cross-linking with polyfunctional cross-linking agents, in which the water content of the polymer particles following surface cross-linking is readjusted to from 3 to 9 wt. %, and this quantity of water may contain inorganic compounds such as metal salts.
Superabsorbent polymers which are contacted in particle form with polyvalent metal salts by dry mixing and are then provided with a specific quantity of a liquid binder such as water or polyols according to WO 98/48857, should have improved gel blocking when absorbing aqueous liquids. The polymer particles may undergo a surface post-cross-linking before this treatment.
In order to minimise the tendency of superabsorbent, post-cross-linked polymer particles to agglomerate as a result of electrostatic charging, WO 98/49221 recommends re-moistening the polymer particles with an aqueous additive solution up to 10 wt. % water. These aqueous solutions may contain monovalent or polyvalent ions or propoxylated polyols. It is also possible to contact the polymer particles with the aqueous additive solution as early as before the surface post-treatment, as a result of which a more uniform distribution of the surface treatment agents should be achieved.
No reference to any possibility, in the post-cross-linking stage, of also dramatically increasing permeability properties, while preserving a high retention capacity and absorption capability for liquid at pressure, is apparent in the prior art described hereinabove.